The onset of a chronic illness during adolescence presents a significant challenge during a period of important biological, psychological, and social changes. Despite the importance and complexity of the adolescent's social world, little is known about the effect of a chronic illness on adolescent social functioning, and even less is known about the process of social adjustment after diagnosis. Inflammatory bowel disease (IBD) is an ideal disease in which to describe the bio-psychosocial pathways between chronic illness and its impact on social functioning during adolescence. IBD is often diagnosed in adolescence and is associated with symptoms of depression, restrictions in social activities, and delayed puberty, all of which may be particularly detrimental to social functioning during this developmental period. Previous research has shown that adolescents with IBD continue to experience social difficulty a year or more after diagnosis, even with only mild disease symptoms, but the mechanisms underlying these associations are unknown. Our goal is to improve the adjustment of adolescents with chronic illnesses by determining which aspects of social functioning are most problematic and by determining how biological and psychological factors contribute to these difficulties. We will recruit adolescents with IBD at the time of diagnosis and match each to a healthy comparison adolescent who attends the same school. Assessments will occur at baseline, 6, 12 and 24 months to examine the processes by which adolescents adjust to the challenges in social functioning. Social functioning will be assessed in several areas (e.g., social behaviors and adjustment) and from several sources (peers, teacher, parent, self). Biological factors will include pubertal timing and disease severity. Psychological factors include depression symptoms and coping strategies, and gender will also be examined. This research will provide new, important information about the bio-psychosocial pathways between chronic illness and its impact on social functioning in adolescence. Public Health Relevance: This research is relevant because having a chronic illness likely affects adolescent social functioning. Identifying their specific social difficulties and what contributes to them is the first step in improving adolescents' lives in an area that is not only important to them, but also has important implications for successful adulthood.